A passive optical network (PON) technology is a point-to-multipoint fiber optic transmission and access technology, and uses a broadcast manner in the downstream and a Time Division Multiple Access (TDMA) manner in the upstream. A PON may flexibly form topological structures, such as tree, star, and bus. With no need of a node device at an optical branch point, only a simple optical splitter needs to be mounted.
FIG. 1 is a schematic structural diagram of a tree topology used by a PON. The topology includes an optical line terminal (OLT) 101 on an office side, an optical network unit (ONU) or optical network terminal (ONT) 103 on a user side, and an optical distribution network (ODN) 102. The meaning of “passive” is that the ODN does not include any active electronic component or electronic power supply, but includes only a passive component, such as an optical splitter.
In a PON system, a direction of transmission from an ONU to an OLT is an upstream direction, in which a TDMA manner is used, that is, upstream data sending is in a burst mode. Different ONUs occupy different upstream timeslots, and multiple ONUs share an upstream link in a time division multiplexing manner. There is a collision avoidance guard interval between each upstream timeslot and a next upstream timeslot. To ensure that an upstream data collision does not occur, a ranging technology needs to be used in upstream sending in order to measure a round trip time of a data signal from being sent in the downstream to being received in the upstream by an OLT, and perform delay compensation accordingly. Therefore, it is ensured that after converging into a common optical fiber, upstream signals of each ONU are inserted into designated timeslots, and neither a collision nor an excessive gap exists between the upstream signals. The ONU further needs to register on the OLT before upstream ranging is performed, and registration and ranging are separate. A specific registration method is as follows. The OLT periodically allocates an upstream registration window, and the ONU sends a registration message in the upstream registration window. The OLT parses the registration information sent by the ONU, and sends acknowledgment information to the ONU if the information is correctly parsed, or the OLT does not perform processing if the information is not correctly parsed, without receiving acknowledgment information after a timeout, the ONU randomly backs off for a period of time, and then sends the registration information again in an upstream registration window for registration until registration succeeds. A specific ranging method is follows. An OLT opens a ranging window for a registered ONU, and performs ranging using a timestamp method. A disadvantage of this technology is that registration and ranging are separate, and at least two upstream windows greater than a propagation delay are needed.
With development of technologies, an orthogonal frequency division multiplexing (OFDM) technology is currently applied to the PON system. However, the foregoing registration and ranging methods cannot be applied to an OFDM-PON system. In an OFDM-PON, a registration message is used only for phase synchronization based on a sampling point, and cannot complete demodulation. Therefore, a special modulation form needs to be set for demodulation of information carried in an OFDM symbol. Ranging in the PON is Media Access Control (MAC) layer ranging based on a timestamp, and a delay between a MAC layer and a physical layer (PHY) is required to be relatively fixed in order to ensure that a ranging result at the MAC layer does not cause a collision at the physical layer. OFDM is in a unit of a symbol, and a delay between a MAC layer and a PHY layer is not fixed. Therefore, in the OFDM-PON system, it is infeasible to perform registration and ranging in a manner used in the PON.